The present invention relates generally to a control valve for use in a vehicle skid control system and, in particular, to a control valve for use in a vehicle skid control system wherein the braking of a predetermined number of wheels of a multi-wheeled vehicle is controlled and at least one braked wheel of the vehicle is not controlled by the skid control system.
Braking a vehicle in a controlled manner under adverse conditions such as rain, snow, or ice generally requires precise application of the brakes by the vehicle driver. Under these conditions, or in panic stop situations, a driver will often apply excessive brake pressure, thus causing the wheels to lock such that excessive slippage between the wheels and the road surface takes place. Wheel lockup conditions can lead to loss of directional stability and, possibly, uncontrolled vehicle spinout.
In a continuing effort to improve the operational safety of vehicles, many companies have been involved in the development of skid control braking systems. While typically such systems are adapted to control the braking of each braked wheel of a vehicle, some systems have been developed for controlling the braking of only a portion of the braked wheels. Examples of prior art skid control systems are disclosed in U.S. Pat. Nos. 3,515,440; 3,870,376; and 3,880,474.
Generally, prior art skid control systems include a central control unit for monitoring the speed of the controlled wheels to determine the deceleration of the controlled wheels. When the brakes of the vehicle are applied and the wheel deceleration of the monitored wheels exceeds a predetermined deceleration threshold, indicating that there is wheel slippage and the wheels are approaching a lockup condition, the central control unit functions to control the application of hydraulic pressure through a suitable valve means to the associated brakes to prevent lockup of the controlled wheels. Typically, the skid control system includes means for cyclically relieving and reapplying pressure to the associated brakes to limit wheel slippage to a safe level while continuing to produce adequate brake torque to decelerate the vehicle as desired by the driver. In these systems, the means for reapplying pressure is generally a separate hydraulic power source.
Despite the tremendous advantages a skid control system can provide in stopping a vehicle in a controlled manner under adverse braking conditions, very few vehicles are actually provided with such control systems. One of the chief reasons for this is that the control units and associated valving of such systems are somewhat sophisticated and expensive, and are therefore typically only found on more expensive vehicles.